This invention relates to current limiter circuits and more particularly to a current limiter that is used in a telephone signalling circuit, e.g., a ring-ground circuit, and it supplies a ground-start signal.
Current limiting, when it was employed, generally consisted of a pair of equal valued series resistors connected in each leg of a telephone circuit. Thus the maximum current drain that could be obtained from the standard -50 volt central office battery was limited by the value of these resistors and the resistance of the connected signalling circuit. In this case, without a current limiter, a switching transistor that is enabled by and receives its base drive from the signalling circuit provides a ground via its emitter-collector path to supply a ground-start signal. Since the collector current of the switching transistor may be considerable in a short loop, a large and expensive high power transistor is required here. This function has also been accomplished with a relay which is even larger, requires more power, and is more expensive.
With the advent of sophisticated control devices in the form of microprocessors, the time-honored signal functions could be readily derived in a very low power miniaturized device. However, it is necessary in the telephone industry to make sure that the new is compatible with the old, thus assuring the integrity of the system.